Post hoc ergo propter hoc (Latin: "after this, therefore because of this") is a logical fallacy (of the questionable cause variety) that states "Since event Y followed event X, event Y must have been caused by event X." It is often shortened to simply post hoc fallacy. It is subtly different from the fallacy cum hoc ergo propter hoc ("with this, therefore because of this"), in which two things or events occur simultaneously or the chronological ordering is insignificant or unknown. Post hoc is a particularly tempting error because temporal sequence appears to be integral to causality. The fallacy lies in coming to a conclusion based solely on the order of events, rather than taking into account other factors that might rule out the connection.

The following is a simple example:

The rooster crows immediately before sunrise, therefore the rooster causes the sun to rise.

"I can't help thinking that you are the cause of this problem; we never had any problem with the furnace until you moved into the apartment." The manager of the apartment house, on no stated grounds other than the temporal priority of the new tenant's occupancy, holds that the tenant's presence has some causal relationship to the furnace's becoming faulty.[1]

In the obsolete theory of spontaneous generation, people once erroneously believed that maggots arose spontaneously from rotting meat, due to observing their presence at the site of rotting meat. Francesco Redi disproved this by using jars with lids permeable to air but not insects, compared to uncovered jars. The flies could only enter the uncovered jars and those were the only ones in which maggots appeared.

The number of vaccines children are receiving is increasing, and the number of children who are being diagnosed with autism at some time after being vaccinated is on the rise. Therefore vaccines must cause autism.